The present invention relates to a system and process for anti-lock control which allow controlled braking with high vehicle deceleration even at maximum vehicle speeds.
In the known anti-lock system, brake-pressure reduction phases are triggered on the wheel brake of a vehicle wheel tending to lock when a sum X.sub.v,h .multidot.Z.sub.v,h +Y.multidot..lambda..sub.v,h of amounts, weighted with the weighting factors X.sub.v and X.sub.h, of the wheel-circumference deceleration Z.sub.v and Z.sub.h and of the brake slip .lambda. of the respective vehicle wheel, related to a reference speed, exceeds a response threshold value standardized for practical reasons at 1; that is to say, the weighting factors X.sub.v,h and Y are determined in such a way that, should the control respond solely as a result of an increased brake slip, without the wheel experiencing any appreciable deceleration, or should the wheel decelerate to an extent corresponding to the locking limit, without there being any appreciable brake slip, the product Y.multidot..lambda..sub.v,h or the product X.sub.v,h .multidot.Z.sub.v,h each has the value 1.
In the anti-lock systems corresponding to that of the German Utility Model G 69 25 179.2 and in developments of this system, the weighting factors X.sub.v,h and Y are predetermined as vehicle-specific constant quantities, as a result of the mutual coordination of which a dynamically stable braking behavior is achieved, a safety value .DELTA.Z.sub.o of a typical amount of 0.3 also being added to the said sum, so that the control commences on the wheel entering an increased deceleration and/or brake slip, before the locking limit is reached.
On production vehicles attaining maximum speeds of around 200 km/h and having an approximately speed-neutral aerodynamic behavior, this method of generating the reference quantity is perfectly suitable, even at maximum vehicle speeds, for obtaining in the anti-lock control mode the highest possible vehicle decelerations in relation to the road conditions and tire quality. It is unsuitable, however, for racing vehicles equipped with aerodynamic aids which at high vehicle speeds, which in the case of a racing vehicle may be up to 350 km/h and more, bring about drastic increases in the axle loads which also have a varying effect on the front axle and the rear axle, the aerodynamic speed-dependent axle-load increase on a modern racing vehicle reaching on the rear axle approximately double the value of that on the front axle.
It has been shown that racing vehicles equipped for test purposes, with an anti-lock system of the above-mentioned type, even when there were brake-pressure distribution controls working as a function of the axle load and therefore allowing a load-related front-axle/rear-axle brake-force distribution, achieved clearly poorer vehicle decelerations than racing vehicles of corresponding design, but without an anti-lock system.
The object of the present invention is, therefore, to improve a process for anti-lock control, to the effect that it allows controlled brakings with high vehicle decelerations, even at maximum vehicle speeds, and to provide an anti-lock system suitable for carrying out that process.
This object has been achieved, where the process is concerned, by reduction of the weighting factors, as a function of vehicle speed v, according to a relation EQU X=X.sub.o -b.multidot.v,
under the secondary conditions that ##EQU1## Z.sub.maxv and Z.sub.maxh satisfying the conditions ##EQU2## in which the quantities Z.sub.blv and Z.sub.blh are formed by the respective relations ##EQU3## in which .mu..sub.v designates the adhesion coefficient on the front axle
.mu..sub.h the adhesion coefficient on the rear-axle the rear-axle load fraction PA0 .chi. the height of the center of gravity related to the wheel base PA0 .phi. the rear-axle brake-force fraction related to the vehicle weight PA0 K.sub.VA the lift (negative lift) coefficient on the front axle PA0 K.sub.HA the lift (negative lift) coefficient on the rear axle PA0 K.sub.w the coefficient of air resistance, and PA0 G the vehicle weight (N).
The method according to the present invention forms the weighting factor X, unitary for the front axle and the rear axle, the consequence of which is a weighing of the wheel-circumference deceleration decreasing with an increasing vehicle speed, in the formation of the reference quantities K.sub.v and K.sub.h to be put in relation to the response threshold of the anti-lock control. This method achieves the result that the control responds when the vehicle wheel subjected to the control reaches at least approximately its best possible deceleration for brake-force transmission under the particular boundary conditions, that is to say achieves a control behavior which, with a stable braking behavior of the vehicle, nevertheless allows the highest possible vehicle decelerations to be utilized according to circumstances, this being considered a great advantage.
The formation of the weighing factor X, corresponding to a linear function of the speed, is highly suitable for a rapid on-line processing of the speed information data and allows a quick reaction of the ABS, this being especially important in racing.
In terms of an anti-lock system for carrying out the process according to the invention, the objects are achieved by an anti-lock system which, as regards its basic design, has an electronic control unit of the anti-lock system which comprises a speed comparator which, when a speed threshold value is exceeded, generates an output signal causing the switch to the brake-force distribution with the higher rear-axle brake-force traction, and a comparator which compares the reference quantity K.sub.h with the response threshold value and which, as soon as the reference quantity K.sub.h reaches the reference value and the control responds in the direction of pressure reduction phase, switches the brake system back to the brake-force distribution linked to the lower rear-axle brake-force fraction.
Another object is that the anti-lock system can be produced especially simply particularly in combination with a brake system with a switchable brake-force distribution.
The relevant switchability of the brake system can be obtained by designing the rear wheel brakes as 4-cylinder brakes, with a pair of cylinders of one wheel brake being combined respectively with a pair of cylinders of the other wheel brake to form a part brake circuit of the rear-axle brake circuit. One of the two part brake circuits is shut off from the brake unit below the threshold value v.sub.s by a value which is moved into its open position by the output signal of the speed comparator. Alternatively, a known master cylinder, with a switchable brake-pressure distributor, can be provided for switching the brake-force distribution.